1993 Buick Lesabre Sudden Engine Vibration

Out-of-balance tires IS the most common suspect, but there are other things to consider. First, the balancer could have been out-of-calibration or had bad bearings. Either of those would not balance the tires correctly. Next, you also have to look for a bent wheel, out-of-balance brake rotor, or in extreme cases, a warped brake rotor. A tire can have a broken belt too. You can usually identify that when it isn't real bad yet by watching for an oscillating steering wheel while you drive very slowly, as through a parking lot.

This is not a wheel bearing issue.

Tire balancers are the machines in the shop used to balance tires. They have a shaft sticking that the wheels are mounted to, then the machine spins the assembly while sensors measure any imbalance. That spinning shaft rides on bearings that can cause rumbling, just like noisy wheel bearings on a car. The machine's sensors can detect that rumbling and confuse it with an imbalance. It will display that as a need to add a wheel weight. Most mechanics don't realize their balancer has bad bearings until they start getting a lot of customers coming back with unsatisfactory results after having their tires balanced. An additional clue the machine needs to be fixed is it will show a need to add a weight, the mechanic does that, spins the assembly again to check the results, and finds it needs a second weight in a different spot, and that might happen a third time.

Most tire balancers need to be calibrated periodically too and most have provisions for doing that in less than a minute or two. We often don't do that until we realize we're having multiple customer complaints.

Bad wheel bearings will cause a buzzing noise similar to an airplane engine. If you wiggle a tire and feel a slight knocking, have the steering and suspension systems inspected at a tire and alignment shop. That looseness can develop in a wheel bearing, but if you don't have that buzzing noise too, it's much more likely the looseness is due to a worn ball joint, tie rod end, or strut.

A visual inspection of brake rotors is similar to saying a person's teeth are okay because you see them smile. There's WAY more to it than that. The only thing a visual inspection will show is if large sections are broken away due to rust, but that almost always occurs on the backside where it's hard to see. Many rotors have bent strips of metal stuffed into one of the cooling slots to balance it. If a weight fell out, you may feel that out-of-balance if it's bad enough.

Rotors are always machined during a brake job to make the two sides perfectly parallel to each other and to make the surfaces perfectly flat to match the new pads. If new pads are just slapped on with no regard to resurfacing the rotors, the pads will not make full contact until they wear down over thousands of miles. What little part does make contact does all the work so it builds up heat really fast. That can cause a rotor to warp. You'll never see that by eye. "Thickness variation" is measured with a micrometer in multiple spots around the rotor. If there is excessive thickness variation, you'll feel that as a brake pedal pulsing up and down when you apply the brakes.

"Lateral run out" is measured with a dial indicator. That means the braking surface of the rotor isn't parallel to its center mounting area so it wobbles as it goes around. A little lateral run out can be tolerated, but if it's excessive, you'll feel that as an oscillation in the steering wheel when braking, and sometimes even when not braking. You may or may not feel that in the brake pedal.

The linings on the brake pads are not as wide as the friction surface on the rotors. Rust builds up on the inner and outer areas where the pads don't make contact. Those linings are never in exactly the same position as on the old pads being removed. That means the edge of the new linings will be riding on a ridge of rust. You might feel that in the brake pedal, but it's more likely you'll hear that as a rough grinding sound, worse when braking.

Thickness variation, lateral run out, and rust ridges are all handled by machining the rotors during a normal brake job, but there is a legal minimum thickness published by all car manufacturers that those rotors can be machined to. There is a second, slightly thinner specification a rotor can be allowed to wear to and still be left on the car. If a mechanic machines your rotors too thin to try to save you money, he can easily end up in court if that is found during a crash investigation, even if the crash was caused by the other person. New rotors today are real inexpensive so no conscientious mechanic is going to risk his reputation or his job by machining a rotor that needs to be replaced. Most rotors cost less than the cost of labor to machine old ones and the cost of cutting bits and maintaining the brake lathe. It's usually less expensive to just sell you a pair of new rotors.

New rotors already have the proper "surface finish" to aid in the new pads seating and matching perfectly. That insures even side-to-side braking to avoid pulling to one side. Old rotors that are machined or worn to different thicknesses will build up heat at different rates, especially during prolonged city driving which is where brakes work the hardest. The "coefficient of friction" changes as temperature rises, and if those temperatures rise faster on one side than the other side, a brake pull can develop suddenly and unexpectedly. A lot of pickup trucks have been having that problem since the mid '90s, and only experienced brake system specialists know what's causing it and how to solve it. All of these problems are caused by brake rotors that look just fine, so a visual inspection has little value.

I'm not an engine performance specialist so I can only suggest how I would approach this on my Chrysler products. I'm pretty sure something similar will apply to your car because GM and Chrysler use the exact same part for idle speed control.

On my cars, the scanner shows the "idle steps" from 0 to 256. That is the setting the Engine Computer has set the idle speed motor to. This isn't a motor as you would think of a spinning motor. The armature is placed to specific positions, and as it slowly rotates, it's connected to a threaded shaft with a pintle valve on the end. Rotating the shaft moves that valve in or out to allow more or less air to bypass the throttle blade. At the same time the computer adjusts how long the injectors pulse open to adjust the amount of fuel. Together those two things control idle speed, but once you press the accelerator pedal even slightly, you are in control, not the computer. If the engine speed is erratic when you're pressing the pedal, that isn't an idle speed problem.

With a properly-running engine, step 32 is a typical setting to find the idle speed motor at. That is displayed on the scanner. With one cylinder misfiring causing engine speed to drop, the computer will bump it up to around step 50. These numbers are just approximate and vary a little constantly. What you're looking for is what the computer is requesting relative to what is actually happening. For example, if idle speed is too high and the idle speed motor is at step 62, the computer is requesting that higher speed in response to something. A good suspect on GM cars is an ambient air temperature sensor or coolant temperature sensor that is disconnected or has a broken wire. The computer will think something is colder than it really is, and that calls for more fuel, just like using a choke on older cars.

If you find idle speed is too high but the computer has the idle speed motor at step 0, it is trying to lower idle speed but without success. A vacuum leak is almost always the cause of that.

We used to run into an idle speed that was too low and would cause stalling at stop signs and hard starting unless you held the accelerator pedal down 1/4". We'd find the idle steps rather high, like 40 to 60. The computer saw it needed to raise engine speed, but it again, wasn't having the expected results. That was a common problem on one engine in particular that I have in two minivans. It was caused by carbon buildup blocking that air passage around the throttle blade. While the idle speed motor was opening up that air valve, no air could get past that blockage. It only took a few minutes to clean that and solve the low idle problem, but I suspect it's due to the better additives in today's fuels, I haven't heard of anyone running into that carbon issue for many years.

Some scanners and some car manufacturers display their idle control status in percent instead of steps. I don't know what is common or typical. What I look at is it will display "target percent", meaning what it expects to be normal, then I compare that to the actual percent. Reading the steps or percent isn't really diagnosing anything. It is simply reading some of the information to get an idea of what types of problems to look for.

Your Engine Computer can also display diagnostic fault codes, but on '95 and older cars, there's only a few dozen potential codes, so the computers aren't too sophisticated. You may have no codes in memory but reading them is always the place to start with running problems. With many newer cars, the people at most auto parts stores will read the codes for you for free. On older Chryslers you can do that yourself by cycling the ignition switch three times. On GM cars, there's two terminals to connect with a paper clip or jumper wire, then you watch the flashing of the Check Engine light to get the code numbers. The connector is under the steering column, attached to the bottom of the dash. Here's the link to the page that tells you how to read the codes and what each one means:

https://www.2carpros.com/articles/buick-cadillac-chevy-gmc-oldsmobile-pontiac-gm-1983-1995-obd1-code-definitions-and-retrieval-method

When they refer to terminals "A" and "B", those are real easy to find in the connector. They're the only two right next to each other in a corner. In the other three corners there will be only one terminal and one empty cavity.

The sensor will cause this code about half of the time. First you should do a visual inspection of the wiring, then check the connector terminals for corrosion and a tight fit between mating pins.

An engine performance specialist would probably know right where to start looking, or would know of a common cause, but for me, my approach would be to connect a scanner with a record feature. While on a test-drive, you press the "Record" button when the problem occurs, then the scanner records a few seconds of sensor data that can be played back slowly later to see what changed. Since the data travels through the scanner's memory, the recording actually begins a couple of seconds before you pressed the button.

There's two things to look for. The first is a momentary loss of signal from a sensor or a sudden change in signal voltage that would CAUSE the hiccup, and the second thing is to find a sensor with a reading that changed as a RESULT of that hiccup. I know how to find the first ones. The second ones are typically oxygen sensor readings. I would need to ask someone else to explain most of those results.

You can't go by oil level. If you filled that spark plug tube all the way to the top, the oil level would go down less than 1/16", and that would still place it between the "Min" and "Max" marks on the dip stick. Keep me up-to-date on your progress.